why does u(x + delta x) = [u(x)+ delta u]?

samuel-choi

why would u(x + delta x) = [u(x)+ delta u] ?

i'm trying to picture it on a graph but it doesn't make sense

ted-s

likely because it is not valid ......but u(x + δx ) ≈ u(x) + δx ( du/dx ) , if du/dx exists